The present invention relates to measuring heads for magnetometers making it possible to measure a magnetic field in which the head is present. It also relates to magnetometers incorporating such a head and which are constituted by a measuring case having a relatively large size connected by a cable to a small head able to measure the magnetic field in narrow spaces, making it possible to arrange magnetic gradient cards.
U.S. Pat. No. 4,112,367 published Sept. 5, 1978 describes a magnetometer utilizing the Faraday effect in thin layers of certain materials such as yttrium iron garnet, called YIG. A light beam produced by a laser is propagated in said magnetometer in a thin layer with a thickness of a few microns. This light beam is propagated in guided manner in accordance with a single mode, e.g. TM. For selecting this mode and bearing in mind the limited thickness of the layer, said light beam must be introduced into the layer by means of a coupling prism. Under the action of the magnetic field in which is present in the thin layer, the propagation mode is converted into a TE mode with a percentage varying as a function of the magnetic field value. To extract the light beam from the layer a birefringent material prism is used, which supplies two angularly separated light beams, one corresponding to the TE mode and the other to the TM mode. The intensity of the beam corresponding to the TE mode is a function of the intensity of the magnetic field in which is plunged the thin layer. As this function is not linear a zero method is used for carrying out the measurement and this consists of compensating the magnetic field to be measured within the thin layer by a regulatable opposing field produced by a coil. To detect this compensation an alternating field is superimposed on this direct field leading to the appearance in the beam corresponding to the TE mode of a modulation which only contains, when compensation has taken place, even harmonics of the frequency of the alternating field due to the fact that the Faraday effect is dependent on the value of the magnetic field and not on its direction. When the compensation is not produced uneven components are detected in the modulated beam and specifically of the fundamental type. These components are used for controlling a servomechanism, which regulates the compensating field to obtain the disappearance of these uneven components.
Although the measuring layer is of small size, the auxiliary means required for the use thereof are much larger and are difficult to adjust. Thus, the input and output prisms necessary for obtaining a coupling of the beam with the layer and for angularly separating the two beams at the output are relatively large and the pressure with which they must be engaged on the layer must be adjusted in a critical manner by means of devices such as screws, which further increase the overall dimensions. Correlatively the assembly must be surrounded by large coils. In addition, monomode propagation requires the use of a high power and consequently large laser as the light source. Finally the angular separation of the beams from the output prism is not very great and involves positioning the detector far enough away to ensure that it is only excited by the single beam selected.
Under these conditions the measuring head incorporating all these members is much larger than would have been expected in view of the small size of the sensitive layer.